Wireless power transfer is generally achieved by electromagnetic field coupling between a power transmitter and a power receiver. A primary electromagnetic field coupling component in the power transmitter generates an electromagnetic field that is picked up by a secondary electromagnetic field coupling component in the power receiver when the power receiver is placed near the power transmitter. The overall power receiving device (containing the power receiver) rectifies, conditions and/or regulates the received power as needed for the functioning of the device.
This power transfer technique is widely used in many electrical devices (typically, but not exclusively, mobile or handheld devices, e.g. cell phones, electric toothbrushes, etc.) for a variety of purposes (e.g. to safely isolate the transmitter from the receiver, to recharge batteries encased within a watertight housing, etc.). The Qi Low Power Specification (System Description, Wireless Power Transfer, Volume I: Low Power, Part 1: Interface Definition, Version 1.0.2, April 2011) of the Wireless Power Consortium (the WPC standard) is an industry standard that sets forth a suitable description for implementing a wireless power transfer system (wireless power transmitter and wireless power receiver) and is incorporated herein by reference. Additionally, the Qi Compliant Wireless Power Transmitter Manager bq500110 data sheet (November 2010—Revised April 2011) and the Integrated Wireless Power Supply Receiver, Qi (Wireless Power Consortium) Compliant bq51010, bq51011 and bq51013 data sheet (April 2011—Revised May 2011), both by Texas Instruments Incorporated, are specific examples of a wireless power transmitter and wireless power receiver, respectively, and are incorporated herein by reference. (Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.)
In a typical wireless power transfer system design (including that of the WPC standard) the power transmitter generates an electromagnetic field through a primary inductor coil. In a power pick-up unit with the overall power receiving device, a secondary inductor coil generates an AC current and voltage from the electromagnetic field. A rectification circuit rectifies the AC current and voltage to a DC current and voltage. A voltage regulator (or an output disconnect or a voltage conditioner, herein referred to as the voltage regulator) regulates the rectified voltage to a relatively constant or stable output voltage that can be used by the overall power receiving device to charge a battery or power its primary function circuitry.
When the power receiver (or just the secondary inductor coil) is within the electromagnetic field generated by the power transmitter, the secondary coil affects the electromagnetic field according to its resonant frequency and, thereby, changes the current through the primary inductor coil. This change in current can be used by the power transmitter to detect a load due to the presence of the power receiver and to receive communications from the power receiver. Controlling the amount of power wirelessly transferred from the power transmitter to the power receiver is thus achieved by sending feedback or error-signal communications from the power receiver to the power transmitter (e.g. to increase or decrease the transmitted power) by means of varying the resonant frequency of the secondary coil and, thus, the effect of the secondary coil on the electromagnetic field. For example, the power receiver may periodically communicate its operating voltage, current and power levels or send commands for corrective actions required from the power transmitter to keep power receiver parameters within desired operating ranges. The power receiver typically communicates these signals, data or commands by modulating the load seen by the power transmitter with a differential bi-phase encoding scheme. The power receiver usually performs this modulation by switching on and off a modulation component (e.g. one or more capacitor, resistor or other dissipative component) that is typically placed before or after the rectification circuit in the power pick-up unit. The switching on and off of the modulation component generally changes the voltage level of the rectified voltage output by the rectification circuit and alters the resonant frequency in the secondary inductor coil. In this manner, the power receiver alters the coupling electromagnetic field between two states that the power transmitter can detect as logic bits in a data bit stream.
In order to explain problems associated therewith, an example of the communication technique is illustrated by a set of graphs shown in FIGS. 1, 2 and 3 as generated by a simulation of a typical prior art wireless power receiver as it communicates generic data or commands to a power transmitter. In this example, two sets of logic bits 100 and 101 (FIG. 1) are transmitted in a bit stream by the power receiver. The sets of logic bits 100 and 101 are encoded into a corresponding bit stream waveform 102 using a differential bi-phase encoding scheme. A fixed set of initial pulses 103 is added to the front of the logic bits 100 and 101 to form a modulation signal 104 that is used to modulate the load (i.e. switch on and off the modulation component) in the power receiver. The power transmitter, thus, senses a series of changes in the coupling electromagnetic field that correspond to the modulation signal 104. When the power transmitter detects the initial pulses 103, it can proceed to properly decode the logic bits 100 or 101 that follow.
FIG. 2 shows a simulated graph 105 of an example rectified voltage generated by the rectification circuit as the modulation component is switched on and off according to the modulation signal 104. During communication time periods 106 (when the logic bits 100 and 101 are being transmitted, typically a few tens of milliseconds), the rectified voltage changes between high and low levels representative of the modulation signal 104 with the encoded bit stream. During pause time periods 107 (when data is not being transmitted and the electromagnetic field is not being altered, typically a few hundreds of milliseconds), the rectified voltage remains at the high level (caused by the modulation component remaining switched off). (An alternative prior art graph could be generated by maintaining the modulation component switched on; in which case, the rectified voltage would remain at the low level during the pause time periods 107.)
Throughout the rectified voltage graph 105, the rectified voltage ripples around the nominal high and low levels. Post-rectification filtering, conditioning and/or regulating generally smooth out the ripples. However, when the average of the rectified voltage (as shown by graph 108, FIG. 3) during the communication time periods 106 is substantially different from the average during the pause time periods, a regulation error typically occurs. The regulation error generally manifests itself as a periodically occurring output voltage ripple and disturbance to the feedback loop.
To compensate for this error, filtering capacitors on the rectification circuit output have to be relatively large due to the relatively large difference in average voltage over the relatively long duration of a cycle of the communication and pause time periods. Additionally, the post-rectification voltage regulator has to operate at a relatively high headroom voltage due to the relatively large difference in average voltage. These compensating measures contribute negatively to the efficiency, form factor and cost of the overall power receiving device. The large filtering capacitors, for example, can take up significant space, which is usually at a premium in a handheld device. Also, the operation of the voltage regulator at the high headroom results in a loss of efficiency, which is highly significant in battery-powered devices.
It is with respect to these and other background considerations that the present invention has evolved.